The present invention relates to the phenomenon know as a fiber fuse, and in particular a method of designing components to limit the propagation of fiber fuses.
Optical power levels in optical transmission systems are generally increasing. This is due to a number of factors.
For instance, optical transmission systems, optical fibers and other optical devices such as polarisation mode dispersion compensation mechanisms and photonic switches, all have attenuation characteristics. Increasing the power of an optical signal provides a better signal to background noise ratio, and allows the signal to be transmitted longer distances over the optical transmission system before optical amplification is required. Advances in laser technology have ensured that higher powered lasers are now more readily and cheaply available, thus allowing a cost effective implementation of high optical power signal generation.
Typical optical transmission systems simultaneously transmit data using a multitude of different wavelengths, each transmission channel having a separate wavelength of light for transmission of the respective optical signal. Increasingly, channels are becoming more closely packed together with regard to wavelength e.g. DWDM (Dense Wavelength Division Multiplexed) systems. Increasing the number of simultaneous optical transmissions at different wavelengths will increase the average optical power being carried by the transmission system.
Many optical systems utilise optical amplifiers comprising optical fiber. An example of this is a Raman amplifier i.e. an amplifier tat utilises the Raman effect. Optical amplifiers of this type normally use relatively high power pump lasers for providing the optical power that is utilised to amplify the optical signal power. Current trends indicate it is increasingly likely that Raman amplifiers will be is in telecommunications systems.
Experiments have indicated that high optical powers propagating through fibers can induce an effect referred to as a xe2x80x9cfiber fusexe2x80x9d. The fiber fuse effect, also termed self-propelled self-focusing (SPSF), is a catastrophic damage mechanism. Electronics letters, Jan. 7, 1988, Vol. 24, No. 1. pages 47-46 by R Kashyap and K J Blow a Electronics letters Jan. 5, 1989, Vol. 25, No. 1, Pages 33-34 by D P Hand and T A Birks describe this phenomena in some detail and describe a fiber fuse damage circuit-breaker, and are incorporated herein by reference.
The fiber fuse effect is believed to be initiated by local heating of the fiber. This can lead to a runway thermal effect which, provided the laser power is sufficient, continues until the fiber core melts. A thermal hock wave is created (visible as a bright spot of side-scattered light) that propagates back along the fiber towards the optical power source. This results in the fiber being permanently damaged and unable to guide light.
Propagation velocity is believed to be of the order of tens of meters per second. A fiber fuse occurring in a telecommunications system could be extremely damaging. Additionally, in systems where optical fiber spans (i.e. typically the length between optical fiber amplifiers) are of the order of 80 kilometers, it will be appreciated that if the fiber fuse is not coined, it has the capacity to damage large lengths of optical fiber. This would require replacement of the damaged fiber. If the fiber fuse is able to propagate into optical processing equipment, such as an amplifier or pump laser, the fiber fuse can result in damage to very expensive network components.
It is therefore desirable to limit the damage caused by fiber fuses. As mentioned above, it has been proposed that the initiation of a fiber fuse results from local heating of the fiber. How is local heating is initiated has not been fully understood, although it has been recognised that a fiber fuse may be initiated at the site of fiber damage, such as a fiber break.
It has been recognised that the propagation of a fiber fuse can be halted by halting the supply of signal power to the fiber, for example by deactivating the laser diodes in the transmitters at the node which acts as the source. It has also been recognised that a beam expander can be used to halt the propagation of the fiber fuse, as described in the article of Electronics letters Jan. 5, 1989, reference more fully above.
The fiber fuse effect is also discussed in the assignee""s copending U.S. patent application Ser. No. 09/544,362, filed Apr. 6, 2000 entitled xe2x80x9cFuse Protectionxe2x80x9d which is incorporated herein by way of reference material.
Until now, there has not been a detailed analysis of the conditions under which a fiber fuse will be initiated, nor the condition which permit the propagation of the fiber fuse to be arrested. There is a need for an understanding of these conditions to enable optimum components to be designed which can halt the fiber fuse travel.
The invention is based on the realisation that a fiber fuse can only be initiated within a fiber when the fiber is carrying power greater than a threshold power. Furthermore, a fiber fuse will not propagate through a fiber (or other component) which has a higher threshold. This threshold power is a function of the fiber characteristics, and this understanding enables components for halting the fiber fuse propagation to be designed, by ensuring that they have a higher power threshold.
Therefore, in a first a aspect, the present invention provides an optical component for use in a transmission system to be positioned within a fiber span for halting the propagation of a fiber fuse along the span, the component comprising a component fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which exceeds the power in the fiber span, the values of the core diameter and the higher mode cutoff wavelength of the component fiber defining the threshold power level.
This component can thus halt any fiber fuse from propagating down the span. The invention is based on the recognition that the threshold power is a function of the core diameter and the higher mode cutoff wavelength of a fiber. For large core diameters, an increase in core diameter increase the threshold power. This is considered to result from reduced xe2x80x9cthermal lensingxe2x80x9d. This is one phenomenon which can be used to explain the propagation of the thermal fuse, and is based on the idea that a local fuse location is imaged to a focal point within the core at an adjacent location, at which a fuse is created. Increasing the core diameter, and therefore the mode field diameter, increases the size of these focal points and thereby reduces the intensity. For small core diameters, it is believed that heat dissipates more readily to the cladding, so that as the core diameter is reduced, the threshold also increases. Thus, the threshold power has a minima value with respect to core diameter (for a fiber of constant cutoff wavelength).
The component may comprise a tapered core fiber, an expanded core fiber or a length of fiber spliced into the transmission fiber. In each case, the characteristics of the component are analysed to provide the desired threshold power level.
According to a second aspect of the invention, there is provided an optical amplifier comprising a rare earth doped fiber, a laser pump source and a coupler for coupling pump light into the doped fiber, wherein an optical component for halting the propagation of a fiber fuse is provided between the pump source and the coupler, the component comprising a component fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which exceeds the power in the rare earth doped fiber, the values of the core diameter and the higher mode cutoff wavelength of the component fiber defining the threshold power level.
This aspect enables protection of the laser pump source of a rare earth doped amplifier, such as an Erbium amplifier.
According to a third aspect of the invention, there is provided an optical network comprising a length of transmission fiber and a Raman pump source providing Raman amplification, a coupler being provided for coupling the output signal from the Raman pump sources to the transmission fiber, wherein an optical component for halting the propagation of a fiber fuse is provided between the Raman pump source and the coupler, the component comprising a component fiber is unable to propagate a fiber fuse when the power is below a threshold power level which exceeds the power in the transmission fiber, the values of the core diameter and the higher mode cutoff wavelength of the component fiber defining the threshold power level.
This aspect enables protection of the laser pump source of a distributed Raman amplifier.
According to a fourth aspect of the invention, there is provided a node for an optical network, comprising an optical transmitter for providing a signal at a specific wavelength onto a transmission fiber, wherein an optical component for halting the propagation of a fiber file is provided between the transmitter and transmission fiber, the component comprising a component fiber which is unable to propagate a fiber fuse when the power is below a threshold power level which exceeds the power in the transmission fiber, the values of the core diameter and the higher mode cutoff wavelength of the component fiber defining the threshold power level.
This aspect enables protection of the laser diodes in the transmitter of a mode.
According to a fifth aspect of the invention, there is provided a method of selecting an optical component for halting the propagation of a fiber fuse through a transmission fiber, comprising:
determining an expected power level within the transmission fiber;
selecting a threshold power level which is greater than the power level;
selecting a component fiber which is unable to propagate a fiber fuse when the power is below the threshold power level, the core diameter and the higher mode cutoff wavelength of the component fiber being selected to define the threshold power level.
This design method provides a component which has a threshold power level which exceeds the maximum power to be transmitted in the system. If power surges are experienced which exceed the expected transmission power levels (the transmission fiber being designed to operate at the transmission power levels without fiber fuses being initiated) then the components provide a safety margin.